A measurement principle that acquires a tomographic image by using coherency of light waves is called optical coherence tomography (OCT) and is currently in practical use as medical apparatuses that acquire tomographic images of retinas (for example, refer to Patent Document 1 and Non-patent Document 1).
In the apparatus, light having low coherency is separated into measurement light and reference light, a measured object is arranged on the measurement light side, a reference mirror is arranged on the reference light side, reflection light from the measured object and reflection light from the reference mirror are superposed to obtain an interference signal, and thereby, a tomographic image is acquired. Therefore, it is possible to obtain higher depth resolution (hereinafter, resolution) as the coherency is lower, that is, the spectral width of a light source is wider.
The apparatuses can be generally classified into time-domain OCT (TD-OCT) and spectral-domain OCT (SD-OCT), and currently, SD-OCT in which mechanical scanning of the reference mirror is unnecessary by spectroscopically analyzing the interference signal when acquiring a tomographic image is the mainstream.
In general, OCT has a feature that the resolution is degraded when group velocity dispersion (hereinafter, dispersion) that is present on a measurement optical path is different from dispersion that is present on a reference optical path. Therefore, when a measured target or an optical system in the vicinity of the measured target has dispersion, there is a disadvantage that the resolution may be degraded due to the effect of the dispersion.
Recently, SD-OCT has been upgraded to develop an intensity-interferometric spectral-domain tomographic imaging apparatus that automatically cancels, even when the dispersion that is present on the measurement optical path is different from dispersion that is present on the reference optical path, the effect of the dispersion (refer to Non-patent Document 2).
The principle of the apparatus is similar to that of SD-OCT; however, the apparatus has a relatively complicated and large configuration in which, for example, two spectroscope devices are required compared to usual SD-OCT.